In the oil production industry, hoist systems are used for drilling and other operations associated with drilling rigs and well service rigs. These rigs can experience certain conditions that result in the main hoist block of the hoist system traveling into too close proximity of the cable support mechanism at the top of the rig supporting the hoist block and equipment and personnel on the rig's deck. The load block exceeding the upper and lower travel limits can result in damage to the rig or hoist equipment and possible injury to the operating personnel. In response to this risk, the field has been motivated to develop means to set both the upper and lower block travel limits and have a shutdown mechanism to automatically stop movement of the hoist block when either limit is reached.
A variety of shutdown mechanisms have been developed in the field to limit hoist block travel to between set-distance points along its travel path. These include simple electromechanical and optical trip switches operated by the physical movement of the hoist block (or some other part of the hoist system) past the switch, which operates to stop the hoist's drawworks. Problems with such simple trip switch travel limiters further motivated the field to develop more sophisticated shutdown mechanisms. These more sophisticated mechanisms often include indirect means of monitoring the position of the hoist block along its travel path. For example, Nield et al. (U.S. Pat. No. 4,334,217) disclose an electronic controller and indicator that monitors the position of the hoist block along its travel path via a linkage to the drawworks drum of the hoist system rather than to the hoist block.
However, there are conditions inherent in the operation of a hoist system other than the mere location of the hoist (or load) block along its travel limit that are important for the hoist system operator to monitor and control. For example, the momentum of the hoist block as it approaches either its upper or lower travel limit impacts what that limit point should be set at in view of the drawwork's braking capability. With drilling operations capable of attaining depths greater than 15,000 feet, static drill string loads of hundreds of tons can be placed on the hoist block. Movement of such high masses can create an extreme condition of momentum that can cause a travel block to exceed a set-distance type travel limit that it otherwise would not—especially a lower limit.
It would be beneficial to the field to have a hoist block travel controller that set travel limits based on “rate of approach” to a “zero momentum point.” This is to say, that the upper and lower limits of the hoist block along its travel path are considered to be points beyond which the hoist block has no momentum (“zero momentum points”), and hence no movement. Therefore, the momentum of the hoist block (the speed and mass of the block including the load it is carrying) and the braking capacity of the drawworks are factored into a function to automatically apply braking to the drawworks at a point within the travel limit of the hoist block depending on the block's mass and rate of approach toward a zero momentum point. Such a shutdown mechanism would permit a maximum travel range to be selected without the same risk of over-travel as in a set-distance type shutdown system.
Another feature of a hoist system that must be monitored, especially in an oil drilling rig, is the ton-mile parameter of the drawworks cable. In a drawworks, the cable wear is influenced by two important factors: the distance traveled by the cable over the sheaves/wheels of the hoist system, and the load supported by the cable. Cable used in such hoist systems has a predetermined life span measured by the product of the number of tons supported times the miles of distance the cable has traveled. This product is designated as the ton-miles parameter of the cable. Exceeding the albeit theoretical ton-mile capacity of a cable can result in failure of the cable with expensive and sometime disastrous results. Also to be avoided is the allowance of too large a safety margin (e.g., caused by an inability to accurately determine the ton-mile wear on a cable), resulting in lost time and added expense when unnecessarily replacing a cable that still has substantial useful life. Therefore, it would be further beneficial to the field to have a multi-function hoist block travel controller that additionally monitored the ton-mile wear parameter of the hoist system's drawworks cable.